Frequency response analysis (FRA) of transformers as a tool for fault detection and location: A review

Faults diagnosis in power transformers has been traditionally based on the insulation resistance measurement, polarisation index, analysis of dissolved gasses in oil, dissipation/power factor measurement, and partial discharges within many other alternatives. Originally, all these techniques presented an offline implementation, that is, with the transformer out of service. Currently, some of them, such as partial discharges measurement or gas analysis (gas chromatography), are carried out online in those cases in which the importance of a machine justifies it. These techniques have been recently complemented with new alternatives, such as frequency response analysis, an offline application technique. At the same time, in recent years, development of online diagnostic strategies has been carried out only based in monitoring of electrical variables. These techniques have the advantage of being economical in relation to traditional ones. Its development are incipient and with high growth potential. This study presents a review of the most important techniques and a critical comparison between them.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/iet-gtd.2017.2086

Power transformers monitoring based on electrical measurements: state of the art

When subjected to short circuit, the transformer winding would suffer from considerable large electromagnetic force and then be distorted to a certain degree, in which cases leakage inductance variation can be employed to analyze and determine the situation and severity of the deformation. In this paper, the transformer leakage inductance identification model is set up on the basis of equivalent circuit equation. Least squares identification method is used to identify leakage inductance. Transverse and longitudinal comparison method are used in this paper to obtain the leakage inductance variation, and then comprehensive judgment of winding deformation is available. Moreover leakage inductance variation based transformer winding deformation monitoring system is introduced, which make it passible to apply this method to practice. In the end, practical applicability is validated by PSCAD simulation results and the data from the field.

Leakage inductance variation based monitoring of transformer winding deformation

Power transformer is one of the major and key apparatus in electric power system. Monitoring and diagnosis of transformer fault is necessary for improving the life period of transformer. The failures caused by short circuits are one of the causes of transformer outages. The short circuit currents induce excessive forces in the transformer windings which result in winding deformation affecting the mechanical and electrical characteristics of the winding. In the present work, a transformer producing only the radial flux under short circuit is considered. The corresponding axial displacement profile of the windings is computed using Finite Element Method based transient structural analysis and thus obtained displacements are compared with the experimental result. The change in inter disc capacitance and mutual inductance of the deformed windings due to different short circuit currents are computed using Finite Element Method based field analyses and the corresponding Sweep Frequency Responses are computed using the modified electrical equivalent circuit. From the change in the first resonant frequency, the winding movement can be quantified which will be useful for estimating the mechanical withstand capability of the winding for different short circuit currents in the design stage itself.

Prediction of Change in Equivalent Circuit Parameters of Transformer Winding Due to Axial Deformation using Sweep Frequency Response Analysis

Power transformers are constantly exposed to mechanical, thermal and electrical stresses during operation. In this paper, the authors propose an improved aging model of power transformers by adding the impact of mechanical deteriorations. In the current practice, the mechanical deformation and dislocation of the windings and core are not sufficiently distinguished as components that influence the aging of the transformer. Hence, the current aging model was expanded with a functional block that contains several typical failures in order to emphasize their impact on the lifetime of transformers and their aging as well.  The authors used the Frequency Response Analysis (FRA) method for the fault detection and location of the mechanical deformations of its active parts. The correlation function is used to determine the level of the detected failure. All presented test results are obtained in real exploitation conditions.

/pdf/fault-detection-using-fra-in-order-to-improve-the-aging-4f0yvg09v8.pdf

Fault detection using fra in order to improve the aging model of power transformer

The failures caused by short circuits are one of the causes of transformer outages. The short circuit currents induce excessive forces in the transformer windings which result in winding deformations affecting the mechanical and electrical characteristics of the winding. In the present work, a 722VA transformer winding is considered for the short circuit force and displacement analyses. The winding is subjected to short circuit current and the respective displacement of the winding is simulated and compared with the measurement. By using Finite element analysis, change in equivalent circuit parameters due to displacement is calculated and corresponding change in sweep frequency response is measured using SFRA. An attempt has been made to predict the displacement profile from change in resonant frequency in case of displaced conditions.

Prediction of transformer winding displacement from frequency response characteristics

The results of breakdown voltages in liquid silicone rubber (LSiR) insulation under stepped polarity reversal voltages at a uniform temperature in a needle-plane electrode system are presented in this article. In the presence of defects such as sharp conductor projections or protrusions in the cable insulation stressed under polarity reversal direct current (dc) voltages, the electric field distribution is far more complex due to nonlinear insulation conductivity and time-dependent field distribution. The electric field distribution and space-charge accumulation are intricate functions of factors such as geometry and nonlinear conduction. In this article, a two-dimensional (2-D) axisymmetric model of a needle-plane geometry is developed using the finite-element method to compute the time-dependent field, space charge, and current density distribution under polarity reversal dc, which apparently did not get much attention until now. Interesting results on the effect of polarity transition time and applied polarity time step on the abovementioned distributions are addressed and presented. The pronounced effects of nonlinear conductivity on the time-dependent field distribution are also demonstrated. Furthermore, from the breakdown experiments conducted on tip-plane electrode system for different tip radii and applied time steps, the results put forward a rational and practical estimate of the breakdown field using the proposed model.

Ajith John Thomas

Papers

Prediction of transformer winding displacement from frequency response characteristics

Breakdown Characteristics of Liquid Silicone Rubber in Needle-Plane Configuration Under Polarity Reversal